The UK's major political parties have shown open hostility or indifference to the demands for much greater gender equality in the devolved parliaments in Scotland and Wales, a study has concluded.

The Electoral Reform Society (ERS) report accuses all the large parties of allowing the issue of equal representation for women to "fester", undermining the ethos which underpinned their foundation in 1999 to improve equality, accountability and wider democracy.

As a result, both legislatures were at risk of being infected by the "yah boo adversarialism" of Westminster, it warned. In a highly critical analysis, the ERS says that "women's representation in Scotland and Wales is stagnant or in decline from previous highs, [because] hostility to measures to correct the imbalance has been allowed to fester, while the prospects of reinstalling effective guarantees appear less and less likely as they work against male incumbents".

In 2003, the Welsh assembly became the first in the world to have equal 50/50 male-to-female representation while Holyrood hit 40% female MSPs that year. After hitting lows in 2007, the proportion of women increased slightly on 2007 at Holyrood to 35%, largely because of unexpected election results, but fell dramatically in Cardiff from 47% to 41.6%.

The ERS report is being published on Thursday, just as the Welsh nationalist party Plaid Cymru is expected to confirm that it has elected its first female leader, Leanne Wood, who will sit in Cardiff Bay along with the Welsh Lib Dems' first female leader, Kirsty Williams.

Meanwhile, in the Scottish parliament, three leadership roles have been fulfilled by women for the first time: Johann Lamont was installed as Labour leader, Ruth Davidson is in charge of the Tories, and Tricia Marwick is the parliament's first female presiding officer, Holyrood's equivalent to the Speaker of the House of Commons.

But the ERS says having more female leaders does not disguise the poor overall figures for women's representation, which in turn rely heavily on Labour and Plaid Cymru, and to a lesser extent, the Greens. They are the only parties with active, if "soft", policies on pushing women candidates.

At Holyrood, the number of female MSPs only increased because a large number of male Labour candidates lost their constituency seats in the SNP rout, leading to more women on the regional lists getting unexpectedly elected. In both Wales and Scotland, 75% of constituency candidates last year were men.

Katie Ghose, the ERS chief executive, said that fact underlined the failure of all the parties to effectively champion equal representation. She said they ought to be trying to outdo each other on this issue. Instead, little effective was being done. "In general, when you look at the devolved institutions in Scotland and Wales, they were trailblazers for women's representation, but progress has now stagnated or is in decline, even with more female representation at the top," Ghose said.

"A woman's place in politics is very much down to the rise and fall of one particular political party [which has to] compensate for the failings of others. "We need to get the parties to be outdoing each other on these issues and being really competitive about getting 50/50 women and men, and we haven't seen that."

Lamont told the Guardian that her party needed to do more, and would be again looking at new positive action policies to improve gender equality, alongside racial equality.

"The Scottish parliament does not match the population it represents," she said. "I think we need to look critically at how we sustain levels of representation and be vigorous. We need to have a long look at positive action measures."

Faced partly by legal challenges, Labour dropped its all-female shortlists and "twinning" of seats after 2003, but in Scotland since has used "zipping" on the regional lists to rank women and men in rotation.

The Scottish National party, Tories and Lib Dems in both countries repeatedly resist pressure to introduce positive action policies in selection procedures. While the SNP won the first ever majority at Holyrood in 2011, with 69 seats, only 28% of those were women.

On Tuesday, the SNP MEP Alyn Smyth voted against proposals in the European parliament to set EU-wide quotas on women in companies and politics, claiming that this should be decided at national not EU level and that quotas were undesirable.

The ERS report is part of a recently launched "Counting Women In" campaign involving it, the Fawcett Society which pushes for equal representation for women in politics, the Hansard Society, the Centre for Women and Democracy in Leeds, and Unlock Democracy.

That campaign has set a target of 50% representation for women in all three British legislatures – Westminster, Holyrood and Cardiff Bay – by 2020. At present, only 22% of MPs in the Commons are women, with no female party leaders and only five out of 23 cabinet posts occupied by women.

It is resisting making set demands about the types of measures all parties should introduce, or calling for a UK-wide law requiring gender equality, arguing that different parties have different cultures and rules.

The campaign has set the Northern Irish assembly at Stormont to one side, partly because its member groups are not active in Northern Ireland but also because the debate about political representation is complicated by the recent conflict.

• This article was amended on 16 March 2012 to clarify the male-to-female percentage representation in the Welsh assembly and at Holyrood. In addition Katie Ghose's name was misspelled as Ghosh and Tricia Marwick as Marwich. These have been corrected.